The present invention relates to a method of continuously measuring, in near real time, the mass concentration of particulates suspended in air, atmosphere or exhausted gas in various environments of work, production and life or point, area and mobile emission sources, using a relatively inexpensive monitor being simplified in handling, which is suitable for high level ambient air quality control and industrial hygiene management and quality control in industrial production process.
As a typical technique of measuring the mass concentration of particulates suspended in an ambient air or an exhausted gas in the above fields, there have been known the following four methods:
(1) Filter Paper Sampling Method
(2) Beta Ray Attenuation Method (hereinafter, referred to as "BAM method")
(3) Quartz Crystal Oscillating Microbalance Method (hereinafter, referred to as "QCM method")
(4) Tapered Element Oscillating Microbalance Method (hereinafter, referred to as "TEOM method") (U.S. Pat. No. 4,391,338 filed Jul. 5, 1983; and Examined Japanese Patent Publication No. 1-45569 filed Oct. 4, 1989)
The filter paper sampling method shown in (1) includes the steps of filtering and sampling particulates suspended in atmospheric air or a gas with a filter paper medium, and weighing the incremental weight of the sampled particulates by a balance, thereby calculating the mass concentration. This is an excellent reference method; however, it has a disadvantage in that the sampling and weighing take lots of time and labor, thereby making it difficult to continuously and automatically measure the mass concentration in real time.
The BAM method shown in (2) includes the steps of filtering and sampling suspended particulates with a roll tape filter paper medium, irradiating beta rays to the sampled particulates, and measuring the change in the attenuation ratio of the beta rays, thereby calculating the mass concentration. This method enables the monitoring for a long period, about three months. However, from the viewpoint of safety, this method uses a radiation source with a low level radioactivity (generally, about 100 .mu.Ci or less) such as a radio-isotope C.sup.14, which causes the following disadvantages. In general, it becomes difficult to collect the above suspended particulates in an amount sufficient to obtain the attenuation ratio of beta rays with necessary measurement accuracy, unless the sampling time is set at a large value, that is, about 1.0 hr. Moreover, the statistical measurement error becomes large, unless the measurement time for the attenuation ratio of beta rays is more than several minutes. For these reasons, it is possible to intermittently and automatically measure the average mass concentration for a long sampling time, that is, about 1.0 hr; however, it is difficult to continuously measure the mass concentration varied in a period shorter than the above sampling time. On the other hand, to improve the above measurement accuracy by enlarging the change in the attenuation ratio of beta rays to the utmost, it is required to reduce the filtration area (generally, about 1 cm.sup.2) and to allow the sampled gas to collectively pass through the reduced filtration area at a high rate (generally, about 15 l/min). The filtration rate of gas is increased to a large extent of about 10 times as much as the allowable filtration rate (10 to 30 cm.sup.3 /s), thereby abnormally increasing the pressure loss of the filter paper medium. As a result, the measurement error tends to be easily generated by an abnormal phenomenon such as the bypass leak of the sampled gas, and the blow of dust through the filter paper medium.
The QCM method shown in (3) utilizes the shear oscillation mode of a circular disk-like AT cut quartz crystal oscillator. The natural frequency of the crystal oscillator is changed depending on the increase in the mass concentration of suspended particulates electrostatically deposited on the surface of the electrode of the crystal oscillator by an electrostatic dust collecting method. The change in the mass of the suspended particulates is detected, thus calculating the mass concentration. This OCM method had the following disadvantages. The principle of this method is based on the assumption that a deposition layer of the dust particles on the surface of the electrode becomes a uniform thin layer; however, actually, in the electrostatic dust collecting method used in this QCM method, the physical properties such as particle size distribution or electric resistivity of dust are changed, and thereby the thickness of the deposition layer becomes non-uniform; and further, since the high frequency oscillation acceleration (several MHz) of the quartz crystal is usually applied to the deposition layer, there occurs the dislodgement and reentrainment of dust from the deposition layer, thereby causing a large measurement error. Moreover, the holding capacity of the amount of the dust deposition on the electrode is extremely small (10 .mu.g), so that it is required to frequently clean the electrode. The electrode is also worn by the above dust deposition, and the expensive quartz crystal must be frequently replaced; and a needle type electrode for electrostatic dust collection is degraded in capacity by the electric wear due to corona discharge, and which must be also frequently replaced. For these reasons, the QCM method is difficult to be used as the continuous monitoring method.
The TEOM method shown in (4) utilizes the so-called cantilever oscillator having a tapered bar element with a through hole which is changed in the axial sectional area, wherein the large diameter end is taken as the fixed end, and the small diameter end is taken as the free end. A filter paper holder is mounted at the free end. The suspended particulates are continuously filtered and sampled while continuously oscillating the filter paper holder, and the change in the reduced natural frequency of the oscillator with time depending on the change in the mass with time is detected, thus calculating the mass concentration. This is an excellent method capable of continuously and automatically measuring the mass concentration with a monitor in near real time. However, since the oscillator and the filter paper are oscillated while usually filtering the gas with the filter paper, the suspended particulates collected on the filter paper are usually applied with a dislodgement force due to the oscillation acceleration, thus causing the reentrainment of dust from the filter paper. Moreover, the filter paper is usually applied with dynamic and static forces due to the gas flow, the frequency is affected by causes other than the increase in mass, thus causing a measurement error. This will be concretely described as follows.
(1) Dislodgement of Particles from Filter Paper of Oscillator, or Mutual Adhesion between Particles PA0 (2) Error of Frequency of Oscillator Affected by Dynamic and Static Pressure Applied to Filter Paper during Gas Sampling
Hereinafter, there will be described the relationship between the adhesive force and the dislodgement force due to oscillation acceleration, of a particle to and from an object or between particles (see H. Krupp and G. Sperling: Theory of Adhesion of Small Particles, J1' of Applied Physics, Vol. 37, No. 11, October 1966, p. 4176 to 4180).
In general, an adhesive force &lt;f&gt; of a particle adhering on an object is due to the so-called van der Waals force fvdw as an inter-molecular force acting between surfaces of points in close proximity to each other, or a capillary condensation force at the contact point. In the measurement method of the present invention, however, the latter may be negligible because the filter paper is usually heated at about 40.degree. C. and the possibility that water or the like is condensed at the contact point between particles is low. As a result, the former, that is, the van der Waals force is dominate, and which is expressed as follows: EQU fvdw=z.sub.0 Pvdw (d.sub.1 d.sub.2)/(d.sub.1 +d.sub.2) (1)
where Pvdw is the van der Waals component of free energy attractive forces at the interface of adhesive area. EQU Pvdw=h.omega./8.pi.z.sub.0.sup.3
where
h.omega.: Lifshitz-van der Walls constant, PA1 Z.sub.0 : adhesive distance between the adherents, PA1 d.sub.1 : particle diameter, and PA1 d.sub.2 : diameter of the object on which the particle adheres. PA1 .rho.: density of particle, PA1 Am: maximum amplitude of free end of tuning fork prongs oscillated with sinewave, and PA1 fs: frequency.
The constant h.omega. differs depending on the physical properties of the particle and the object, and is in the range of about 0.5.about.9 eV=0.5.about.9.times.10.sup.-12 erg. Z.sub.0 is about 1 .ANG.=10.sup.-8 cm.
On the other hand, the dislodgement force fs applied to a particle adhering on an oscillated object due to oscillation acceleration is expressed as follows: EQU fs=(.pi.d.sub.1.sup.3 /6).rho.Am(2.pi.f).sup.2 ( 2)
where
Eventually, from the above two expressions (1) and (2), the relationship between the critical frequency fc in generation of dislodgement and dust particle diameter d.sub.1 is expressed as follows: EQU fc=[z.sub.0 Pvdwd.sub.2 /(d.sub.1 +d.sub.2)/20.67 d.sub.1.sup.2 .rho.Am].sup.1/2 (3)
Assuming that z.sub.0 =10.sup.-8 cm, Pvdw=10.sup.8 dyne/cm.sup.2, the apparent diameter of the filter d.sub.2 =2 .mu.m, the density of dust particle .rho.=2 g/cm.sup.3, and the maximum amplitude at the free end of the oscillator Am=30 .mu.m, the relationship between the dust particle diameter d.sub.1 and the critical frequency fc in generation of dislodgement is calculated on the basis of the above expression (3), which gives the result shown in Table 1.
TABLE 1 ______________________________________ d.sub.1 (.mu.m) 1 5 10 ______________________________________ fc (Hz) 3,094 3,024 1,155 ______________________________________
The frequency of the oscillator type mass microbalance is in the range from about several hundreds to several thousands Hz, and the diameter of the target dust particle is in the range of about 10 .mu.m or less. As a result, it is revealed that comparatively coarse particles each having a particle diameter of several .mu.m among the dust particles collected on the oscillated filter paper tend to be dislodged and reentrained.
In the case that the resonance frequency of an oscillator during stoppage of sampling is approximately 2,000 Hz, by performing the gas sampling at a rate of about 1.5 l/min using a glass fibrous filter having an effective area of about 1 cm.sup.2 there occurs a pressure loss of about several tens mmH.sub.2 O, resulting in the generation of an error of about 0.01 to 0.05 Hz. This error corresponds to the mass change of several .mu.g, and therefore, it is not negligible.